An open secret is something which is widely known to be true amongst people, but which is denied officially, for that very reason, providing sources is more usually than not possible for an open secret.
I have no idea if an open secret is like the open mystery of Ephesians 3:9,10.
Indeed. That seems like a loaded cultural reference point considering the majority of the world (including Christendom) do fine not keeping up with bronze age letters as jargon. Maybe they mean an open secret like the Panama papers?
This is the literal definition of an open secret: https://twitter.com/neeneinnyetno/status/1655217749495361536. It is common knowledge, even in the deep midwest where I am, that there is no construction happening anywhere which doesn't have a large undocumented labor component.
I have no idea if an open secret is like the open mystery of Ephesians 3:9,10.